Heterojunction bipolar transistor and semiconductor integrated circuit device using the same

Abstract

A heterojunction bipolar transistor has a raised breakdown voltage and restrains the rising characteristic of IC-VCE characteristic from degrading. The collector region includes first, second, and third collector layers of semiconductor. The first collector layer is made of a doped or undoped semiconductor in such a way as to contact the sub-collector region. The second collector layer is made of a doped or undoped semiconductor having a narrower band gap than the first collector layer in such a way as to contact the base region. The third collector layer has a higher doping concentration than the second collector layer in such a way as to be located between or sandwiched by the first collector layer and the second collector layer.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates generally to heterojunction semiconductor devices. More particularly, the invention relates to a heterojunction bipolar transistor having a raised or improved breakdown (withstand) voltage, and a semiconductor integrated circuit device using the same.[0003]2. Description of the Related Art[0004]To improve the collector-to-emitter breakdown voltage of heterojunction bipolar transistors during operation, it is important to suppress or prevent avalanche breakdown in the collector region. To realize this, an improved structure has been developed and disclosed, where a semiconductor layer having a low or small impact ionization coefficient is inserted into a high electric-field part of the collector region. This structure is disclosed in, for example, the Japanese Non-Examined Patent Publication No. 7-16172 published in 1995.[0005]FIG. 1 shows schematically the energy band diagram of a prior-art...

AI Technical Summary

Benefits of technology

[0030]With the heterojunction bipolar transistor according to the first aspect of the invention, in the collector region, the third collector layer made of semiconductor of the first conductivity type is provided between the first collector layer (which is made of semiconductor of the first conductivity type or undoped semiconductor) and the second collector layer (which is made of semiconductor of the first conductivity type or undoped semiconductor, also). Thus, the potential at the interface between the first collector layer and the third collector layer is lowered or reduced compared with the case where the third collector layer is not provided. This means that no potential barrier is formed at the interface in question. As a result, the rising characteristic of the collector current-collector voltage characteristic is restrained from degrading.

[0031]The potential value or level of the third collector layer, which is located between the first and second collector layers, is changed by changing its doping concentration. This means that the ratio of the electrical potentials (or voltages) applied to the first and second collector layers can be adjusted, in other words, the ratio of the electric fields in the first and second collector layers can be adjusted. Thus, the electric fields in the first and second collector layers can be well assigned in such a way as to accord with the ratio of the impact ionization coefficients of the first and second collector layers. As a result, avalanche breakdown is effectively restrained from occurring in the first and second collector layers, thereby raising or improving the breakdown voltage.

Problems solved by technology

If so, the high-frequency characteristics of the transistor will deteriorate because of the accumulation and retention effect of carriers caused by the conduction band discontinuity.

However, the above-described prior-art energy band structures shown in FIGS. 2 to 4 have the following problems.

As a result, a problem that the breakdown voltage decreases arises.

As a result, there arises a problem that avalanche breakdown is likely to occur in the layer 116a.

This means that the breakdown voltage is likely to lower.

Moreover, even if the band gap discontinuity is apparently relaxed, this is unable to be really eliminated.

Thus, the energy band structure of FIG. 3 is unnecessary.

However, if so, there arises a problem of degradation of the rising characteristic of the collector current-collector voltage characteristic.

As a result, there arises a problem of degradation of the rising characteristic of the collector current-collector voltage characteristic, as well.

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